Mn.sub.3 O.sub.4 has been produced by sintering manganese (II) oxide in the presence of oxygen. Manganese (II) oxide may be produced by heating manganese (II) carbonate, or by heating manganese dioxide.
These prior methods for the preparation of Mn.sub.3 O.sub.4 require high temperatures to sinter the manganese (II) oxide. In addition, it is obviously inefficient to reduce manganese dioxide to manganese (II) oxide, and then re-oxidize manganese (II) oxide to produce Mn.sub.3 O.sub.4.